Retardation films are mainly used as a member to expand a viewing angle of a liquid crystal display or a constituent member of a circular polarizer.
As the retardation film, polycarbonate is disclosed in Patent Document 1. However, polycarbonate has a problem of retardation irregularity in a peripheral portion of a screen, which is called “frame defect”, since it has large photoelastic coefficient. This problem becomes obvious as a screen of the liquid crystal display becomes large, and the retardation film having a small photoelastic coefficient is required.
A cyclic polyolefin resin is disclosed in Patent Document 2. Since the cyclic polyolefin resin has excellent properties such as a small photoelastic coefficient and low water absorption, it is widely used as a retardation film, but it is expensive and there is a need for a low cost retardation film.
On the other hand, since a polyester film is highly transparent and low cost, it is widely used. Application of this film to the retardation film is investigated as disclosed in Patent Document 3, but since it has a large photoelastic coefficient and retardation characteristics are highly sensitive to stretching conditions, it was difficult to control the retardation properly. Polyester having small birefringence is disclosed in Patent Document 4, but it has a problem that the photoelastic coefficient is large. Further, polyester having small birefringence is disclosed in Patent Document 5, but since a glass transition temperature is low, there is a problem that the retardation changes in the production process and the use environment of the liquid crystal display. In addition, polyester having small birefringence is disclosed in Patent Document 6, but the photoelastic coefficient and the wavelength dispersion, which are necessary for the retardation film, have not been investigated, and it has a problem that a film forming property is defective and a uniform film cannot be obtained.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2001-318233    [Patent Document 2] Japanese Unexamined Patent Publication No. 2004-151573    [Patent Document 3] Japanese Unexamined Patent Publication No. 2000-162419    [Patent Document 4] Japanese Patent Publication No. 2843215    [Patent Document 5] Japanese Patent Publication No. 2854796    [Patent Document 6] Japanese Patent Publication No. 3331121